Problem 40 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is true about ideal solutions? (a) the volume of mixing is zero (b) the enthalpy of mixing is zero (c) both \(\mathrm{A}\) and \(\mathrm{B}\) (d) none of these

Step-by-Step Solution

Answer

(c) both (a) and (b)

Step 1: Understanding Ideal Solutions

Ideal solutions are those where the intermolecular interactions between different components are equal to the interactions in the pure components. This means that the behavior of the mixture closely resembles an ideal system.

Step 2: Analyzing Volume of Mixing

In ideal solutions, the volume of mixing is zero because no expansion or contraction occurs when components are mixed. The total volume of the solution is simply the sum of the volumes of its components.

Step 3: Analyzing Enthalpy of Mixing

The enthalpy of mixing in ideal solutions is also zero. This is due to the fact that no heat is absorbed or released during the mixing process because the forces of interaction are unchanged.

Step 4: Conclusion: Identifying the True Statement

Since both the volume of mixing and the enthalpy of mixing are zero in ideal solutions, the correct statement is that both volume and enthalpy of mixing are zero. Therefore, options (a) and (b) are both correct, making option (c) the true statement.

Key Concepts

Volume of Mixing

The concept of the "volume of mixing" is crucial in understanding ideal solutions. When two substances mix together, you might wonder if the total volume changes. In an ideal solution, it doesn't.

This means the final volume is simply the sum of the individual volumes of each component before mixing. There's no volume increase or decrease. This is because ideal solutions exhibit no expansion or contraction upon mixing.

In simple terms:

• No additional spaces or voids are created between molecules.
• Molecules of one component fit perfectly with those of another.
• Volume after mixing = Volume of component 1 + Volume of component 2.

Thus, if you're dealing with an ideal solution, you can expect that the volume of mixing is exactly zero! This characteristic is part of what makes an ideal solution so predictable and straightforward.

Enthalpy of Mixing

In the realm of chemistry, "enthalpy of mixing" refers to the heat change that occurs when two substances are mixed. For ideal solutions, this value is zero. But why is this the case?

When two substances mix ideally, the energy needed to break intermolecular forces in the pure components is equal to the energy released when new interactions form. There's no net energy change—meaning no heat energy is absorbed or released.

Key points:

• There is neither heating nor cooling upon mixing.
• The process is energetically neutral because old interactions are replaced seamlessly by new ones.

This lack of heat change is what we mean when we say the enthalpy of mixing is zero, a hallmark of ideal solutions.

Intermolecular Interactions

Intermolecular interactions are the forces of attraction or repulsion between molecules. These interactions play a crucial role in defining the behavior of solutions. In an ideal solution:

• The intermolecular forces between different species are the same as those within the pure components.
• No new or unusual interactions occur upon mixing, ensuring that the properties of each component remain unchanged.
• Behavior mimics that of "ideal" gases, where molecules don't influence one another.

Because of this equality in interactions, no additional energy changes take place upon mixing. This is why both the volume and enthalpy of mixing are zero. Understanding these concepts highlights why ideal solutions are particularly simple models in chemistry.